This invention relates generally to the rendering of latent electrostatic images visible. More particularly, the invention relates to non-interactive or scavengeless development systems wherein taut wires are utilized for liberating toner particles from a donor member.
The invention can be utilized in the art of xerography or in the printing arts. In the practice of conventional xerography, it is the general procedure to form electrostatic latent images on a xerographic surface by first uniformly charging a photoreceptor. The photoreceptor comprises a charge retentive surface. The charge is selectively dissipated in accordance with a pattern of activating radiation corresponding to original images. The selective dissipation of the charge leaves a latent charge pattern on the imaging surface corresponding to the areas not exposed by radiation.
This charge pattern is made visible by developing it with toner. The toner is generally a colored powder which adheres to the charge pattern by electrostatic attraction.
The developed image is then fixed to the imaging surface or is transferred to a receiving substrate such as plain paper to which it is fixed by suitable fusing techniques.
The present invention is especially useful in highlight color printing systems where the use of non-interactive development systems are desired. One form of highlight color imaging is described in U.S. Pat. No. 4,078,929 issued in the name of Gundlach. The patent to Gundlach teaches the use of tri-level xerography as a means to achieve single-pass highlight color imaging. As disclosed therein the charge pattern is developed with toner particles of first and second colors. The toner particles of one of the colors are positively charged and the toner particles of the other color are negatively charged. In one embodiment, the toner particles are supplied by a developer which comprises a mixture of triboelectrically positive and negative carrier beads. The carrier beads support, respectively, the relatively negative and relatively positive toner particles. Such a developer is generally supplied to the charge pattern by cascading it across the imaging surface supporting the charge pattern. In another embodiment, the toner particles are presented to the charge pattern by a pair of magnetic brushes. Each brush supplies a toner of one color and one charge. In yet another embodiment, the development systems are biased to about the background voltage. Such biasing results in a developed image of improved color sharpness.
In highlight color xerography as taught in the '929 patent, the xerographic contrast on the charge retentive surface or photoreceptor is divided into three levels, rather than two levels as is the case in conventional xerography. The photoreceptor is charged, typically to -900 volts. It is exposed imagewise, such that one image corresponding to charged image areas (which are subsequently developed by charged-area development, i.e. CAD) stays at the full photoreceptor potential (V.sub.CAD or Vddp). The other image is exposed to discharge the photoreceptor to its residual potential, i.e.V.sub.DAD or Vc (typically -100 volts) which corresponds to discharged area images that are subsequently developed by discharged-area development (DAD) and the background areas exposed such as to reduce the photoreceptor potential to halfway between the V.sub.CAD and V.sub.DAD potentials, (typically -500 volts) and is referred to as Vwhite or V.sub.W. The CAD developer is typically biased about 100 volts closer to V.sub.CAD than Vwhite (about -600 volts), and the DAD developer system is biased about 100 volts closer to V.sub.DAD than Vwhite (about -400 volts).
The viability of printing system concepts such as tri-level, highlight color xerography requires development systems that do not scavenge or interact with a previously toned image.
In a non-interactive single component development system of the type contemplated, a controlled toner cloud is generated by the application of an AC voltage to taut wires in self-spaced contact with the toner layer on the donor roll. If the wires contact the donor roll with excessive force, toner could be physically removed from the donor roll and the donor roll coating could undergo mechanical wear and/or electrical breakdown. A gentle attractive force between the wire and the roll is required at all points along the roll and is provided in part by an electrostatic attraction due to the AC voltage. To accommodate roll runout, the wire mounting method must allow the wires to always follow the roll surface. With prior art mounting methods, the taut wires are attached to end blocks at mounting points approximately 125 .mu.m below the surface of the donor. For mounting points.about.10 mm beyond the ends of the donor roll, the reflection of the wire is about 1.degree.. For a wire tension of 500 gm, the corner of the donor roll must support a load of 8 gm which is sufficient to remove toner near the roll ends. This load can also cause wear of the wire and donor coating and cause electrical failure if electrical arcing between the wire and donor occurs. Such a mounting method also makes it difficult to position spacer rollers near the ends of the donor to control the roll-to-receiver spacing.
The use of taut wires for liberating toner from the surface of a donor member and methods of mounting such wires are known in the prior art. For example, U.S. Pat. No. 5,031,570 granted to Hays et al on Jul. 16, 1991 and assigned to the same assignee as the instant application discloses a scavengeless development system for use in highlight color imaging. AC biased electrodes positioned in close proximity to a magnetic brush structure carrying a two-component developer cause a controlled cloud of toner to be generated which non-interactively develops an electrostatic image. The two-component developer includes mixture of carrier beads and toner particles. By making the two-component developer magnetically tractable, the developer is transported to the development zone as in conventional magnetic brush development where the development roll or shell of the magnetic brush structure rotates about stationary magnets positioned inside the shell.
U.S. Pat. No. 4,868,600 granted to Hays et al on Sep. 19, 1989 discloses a scavengeless development system in which toner detachment from a donor an dthe concomitant generation of a controlled powder cloud is obtained by AC electric fields supplied by self-spaced electrode structures positioned within a development nip. The electrode structure is placed in close proximity to the toned donor within the gap or nip between the toned donor and image receiver, self-spacing being effected via the toner on the donor. Such spacing enables the creation of relatively large electrostatic fields without risk of air breakdown.
U.S. Pat. No. 5,010,367 granted to Dan A. Hays on Apr. 23, 1991 discloses a scavengeless/non-interactive development system for use in highlight color imaging. To control the developability of lines and the degree of interaction between the toner and receiver, the combination of an AC voltage on a developer donor roll with an AC voltage between toner cloud forming wires and donor roll enables efficient detachment of toner from the donor to form a toner cloud and position one end of the cloud in close proximity to the image receiver for optimum development of lines and solid areas without scavenging a previously toned image. The wires are supported by the tops of end blocks. The wire extremities are attached so that they are slightly below a tangent to the surface to the donor roll.
U.S. Pat. No. 5,124,749 granted to Jan Bares on Jul. 23, 1992 discloses an apparatus in which a donor roll advances toner to an electrostatic latent image recorded on a photoconductive member. A plurality of electrode wires are positioned in the space between the donor roll and the photoconductive member. The electrode wires are electrically biased to detach the toner from the donor roll so as to form a toner cloud in the space between the electrode wires and photoconductive member. Detached toner from the toner cloud develops the latent image. A damping material is coated on a portion of the electrode wires. The damping material damps vibration of the electrode wires.
Although AC biased wires in contact with a toner layer on a donor roll have proven to be effective for achieving non-interactive xerographic development, the methods of mounting such wires needs improvement since they are difficult to mount in a consistent reproducible manner. Furthermore, the tensioned AC biased wires in self-spaced contact with the toned roll tend to vibrate which can cause nonuniform solid area development.